Conventionally, there is known a method for producing a three-dimensionally shaped object by irradiating a light beam on a powdery material, which method is usually referred to as a “powder sintering and laminating method”. In this method, the three-dimensionally shaped object is produced by repeating (i) a step of irradiating a light beam on a specified portion of a powder layer to sinter or melt the same into a solidified layer and (ii) a step of placing a new powder layer on the solidified layer and irradiating the light beam on a specified portion of the new powder layer to form another solidified layer (see, e.g., Japanese Patent Laid-open Publication Nos. H1-502890 and 2000-73108). In case where a metal powder is used as the powdery material, the three-dimensionally shaped object thus produced can be used as a mold for molding a plastic article. In the event that a resin powder is used as the powdery material, the three-dimensionally shaped object thus produced can be used as a plastic article. This production method enables a three-dimensionally shaped object of complex shape to be produced within a short period of time.
In order to avoid oxidization of the three-dimensionally shaped object, the production thereof is performed within a chamber kept in a specified inert atmosphere. Installed inside the chamber are a powder layer forming unit, a substrate on which the powder layer and/or the solidified layer are placed, and so forth. A light beam irradiating unit is installed outside the chamber. The light beam emitted from the light beam irradiating unit is irradiated on a specified portion of the powder layer through a light transmission window of the chamber.
With the light beam irradiating unit shown in FIG. 1, the light beam L is emitted from a light beam oscillator 30 and then scanned on an arbitrary position of the powder layer by a scanner mirror such as a galvano-mirror 31 or the like. Thus the powdery material is sintered or molten into a solidified layer. Although the scanner mirror functions to reflect the light beam, the reflectivity thereof is not equal to 100% in practice (but is, e.g., about 90% to 98%). In other words, several percent of the thermal energy of the light beam incident on the scanner mirror are absorbed by a mirror unit. For that reason, the heat thus absorbed is transferred from the mirror unit to a scanner body (including, e.g., a mirror drive unit and a mirror control unit), resulting in an increase in the temperature of the scanner body. As a consequence, the scanner body goes through mechanical displacement (or deformation), which reduces the light beam irradiation accuracy.
In this regard, the mirror unit makes reciprocating swing movement about its drive shaft at a high speed during the course of scanning the light beam. (As shown in FIG. 2, the mirror unit 61 makes reciprocating swing movement, e.g., at a high speed of about 15 to 18 degrees/sec with a maximum swing angle of about ±15 degrees.) This requires the mirror to be lightweight. In other words, the mirror unit 61 needs to have a relatively small size, which means that the absorbed heat is easily transferred to the scanner body 62 (see FIG. 2). More specifically, the smaller the size of the mirror unit 61 becomes, the smaller the thermal capacity thereof is. Thus the temperature of the mirror unit 61 is apt to be increased under the influence of the external heat, allowing the heat to be transferred to the scanner body 62 with ease. In case where a metal powder is used as the powdery material, the energy of the light beam is high. (For example, the maximum output power of a carbon dioxide laser is kept as high as about 500 W.) This means that the influence of the absorbed heat becomes great, eventually increasing the possibility of reduction in the irradiation accuracy of the light beam.